Normally an extrusion of thermoplastic synthetic resin material, i.e. a plastic, can be formed by plastifying the synthetic resin material in a plastifying unit which consists of an elongated worm housing, one or more worms rotatable about the worm axis or the worm axes, an inlet at one end of the plastifier chamber and an outlet at the opposite end for the material to be plastified and the plastified or liquefied stream of synthetic resin, respectively. The plastifying unit, generally referred to as an extruder, can be followed by an extrusion die having an orifice from which the profiled body or extrusion emerges.
The reference to a profiled body is, for the purposes of this disclosure, intended to cover any extruded body having a shape imparted by the extrusion orifice or a forming die provided downstream thereof and can include polygonal, circular or compound cross section bodies, solid bodies as well as hollow bodies, and structural shapes of all sorts. They may be, for example, profiles used in the fabrication of windows (window-structural shapes) or bodies which are extruded around wires or cores as in the case of cables as outer sheaths or the like, etc.
When two or more extrusions are simultaneously produced from the extrusion die or shaping member at the discharge side of the extruder, it is found that there are limits to the quality product which can be produced. These limitations result from the fact that certain speeds of extrusion of the plastic through the apparatus cannot be exceeded because there are limits of the cooling speed. For given maximum length of the extrusion apparatus, because of the poor heat conductivity of the plastic, there are extrusion speed limitations since the extruded body at the outlet side of the extrusion orifice must be such that the body will retain its shape against flow under the effect of gravity or will not further deform.
There are indeed extruders with high outputs but in many cases these have other drawbacks. As a result, when increased output is desired, it is generally necessary to provide two or more extruders. When two bodies are simultaneously extruded from a single apparatus, control of the quality of the respective extrusion has been a problem.